Starting method for a piston compressor and piston compressor

ABSTRACT

In an effective method for starting a piston compressor which is easy to implement, and a piston compressor suitable for carrying out the method, there is provision, in a resetting phase, for driving a piston of the piston compressor in the direction of a compression point of the piston position by the application of a resetting drive moment. The resetting drive moment being maintained until the piston has reached a starting position by overstepping a steady-state point as a result of displacement of fluid out of the compression space, formed between the piston and a corresponding pressure cylinder, through at least one leakage point. In a subsequent acceleration phase, the piston is then accelerated out of the starting position into an operational direction of rotation with a starting drive moment.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to a method for starting a piston compressor. Theinvention relates, furthermore, to a piston compressor operatingaccording to the method.

A piston compressor conventionally has at least one piston which can bemoved in a pressure cylinder by a motor in such a way that the volume ofa compression space formed between the pressure cylinder and the pistonis periodically expanded and compressed in the course of the pistonmovement. During the expansion phase, in this case a compressible fluid,in particular a refrigerant such as R600a, for example, is sucked intothe compression space through an inlet valve provided in the pressurecylinder. During the subsequent compression phase, the sucked-in fluidis compressed and expelled under increased pressure through an outletvalve of the pressure cylinder. The piston position which identifies thetransition from a compression phase into a subsequent expansion phase isdesignated below as the compression point. In a similar way to this,that piston position which identifies the transition between anexpansion phase and a subsequent compression phase is designated as theexpansion point.

The designation “piston compressor” relates both to areciprocating-piston compressor and to a rotary-piston or rolling-pistoncompressor. Where a reciprocating-piston compressor is concerned, apiston is movable linearly in the pressure cylinder. The piston of sucha reciprocating-piston compressor is conventionally driven by a rotatingeccentric. Where a rotary-piston cylinder is concerned, the piston, bycontrast, is driven in rotation directly via a drive shaft and rotateswith respect to the pressure cylinder. Without restriction to one ofthese two forms of construction, the rotary position of the drive shaftor of the eccentric is adopted below as a measure of the pistonposition, so that reference to a specific piston position of areciprocating-piston compressor is also in the form of a given anglevalue. The sequence of an expansion phase and of a subsequentcompression phase, to which, as a rule, a full rotation of the driveshaft or of the eccentric through 360° corresponds, is designated as thework cycle of the piston compressor.

When a piston compressor is in operation, a load moment occurs whichfluctuates periodically with the work cycle and which, as a rule, passesthrough a pronounced maximum (designated below as the load peak) in thecourse of the compression phase. It is customary to configure anelectronically regulated piston compressor in such a way that themaximum drive moment of the motor undershoots the maximum value of theload moment occurring during the load peak. In operation, the load peakis overcome by the inertia of the piston compressor, that is to say bythe kinetic energy stored in a flywheel mass.

When a piston compressor is started, the momentum required for thispurpose is not available, so that a usual piston compressor often cannotbe started by its inherent force, or at least not from every pistonposition. In order to increase the momentum travel and thus makeself-starting easier, the piston of the piston compressor is sometimesrotated backwards, opposite to an operational direction of rotation,before the actual starting operation. In a conventional pistoncompressor, the backward rotation of the piston is terminated at thelatest in a piston position in which the maximum drive torque of themotor corresponds to the load moment opposing a further compression.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a startingmethod for a piston compressor and a piston compressor which overcomesthe above-mentioned disadvantages of the prior art devices and methodsof this general type, by which an improved starting behavior is achievedin a simple way in a piston compressor. The piston position isdesignated below as the steady-state point.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a method for starting a pistoncompressor, which includes during a resetting phase, driving a piston ofthe piston compressor in a direction of a compression point of a pistonposition by applying a resetting drive moment. Maintaining the resettingdrive moment until the piston reaches a starting position byoverstepping a steady-state point as a result of displacement of fluidout of a compression space, formed between the piston and acorresponding pressure cylinder, through at least one leakage point.During an acceleration phase, accelerating the piston out of thestarting position into an operational direction of rotation with astarting drive moment.

Accordingly, there is provision, first, in a resetting phase, fordriving the piston of the piston compressor in the direction of acompression point of the piston position by the application of aresetting drive moment, and for maintaining the resetting drive momentuntil, by overshooting a steady-state point, the piston has reached astarting position. In this case, use is made in a controlled way of atleast one leakage point (outlet) of the piston compressor, through whichthe fluid compressed in the course of the resetting phase escapes fromthe compression space, so that the piston position successivelyapproaches the starting position beyond the steady-state point. In anacceleration phase following the reaching of the starting position, thepiston is then accelerated out of the starting position in anoperational direction of rotation by a starting drive moment. The outletpoint which is used is, in particular, the piston/cylinder play alwayspresent as a consequence of construction in a piston compressor.

By the method described above, a particularly long acceleration travelfor absorbing the kinetic energy required for overcoming the load peakis achieved in a simple way in a piston compressor. The drive system ofthe piston compressor operating by the method according to the inventioncan thereby be configured for a comparatively low maximum drive torque.As a result, production costs are saved to a decisive extent.

The resetting drive moment is preferably directed opposite to theoperational direction of rotation, so that, during the resetting phase,the piston compressor is rotated backwards opposite to the operationaldirection of rotation. This is expedient particularly when, as a resultof the type of construction of the piston compressor, a lower loadmoment and/or a higher leakage rate occur(s) during backward rotationthan during corresponding forward rotation. Particularly where areciprocating-piston compressor is concerned, the output or leakage rateand the profile of the load moment behaves, as a rule, symmetricallywith respect to a reversal in the direction of rotation. In this case,during the resetting phase, the piston may also be driven equivalentlyin the operational direction of rotation.

In order to achieve as long an acceleration travel as possible duringthe acceleration phase, the starting position is preferably selectedsuch that it lies in the vicinity of the compression point or slightlyprecedes the latter, as seen in the operational direction of rotation.Expediently, the starting position lies, in particular, in an angularrange of +/−30° about the compression point.

Accordingly, the piston compressor contains a piston movable in apressure cylinder by a drive system. The drive system contains anelectric motor active on the piston via a drive shaft and an electroniccontrol unit activating the motor. The control unit is in this caseconfigured for activating the motor according to the method describedabove. The control unit is in this case configured, in particular, foractivating the motor during the resetting phase in such a way that thepiston is driven in the direction of the compression point under theaction of the resetting drive moment, and for maintaining the resettingdrive moment until the piston has reached a starting position. Thecontrol unit is configured, furthermore, to activate the motor duringthe acceleration phase in such a way that the piston is accelerated outof the starting position into the operational direction of rotationunder the action of the starting drive moment.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin a starting method for a piston compressor and a piston compressor, itis nevertheless not intended to be limited to the details shown, sincevarious modifications and structural changes may be made therein withoutdeparting from the spirit of the invention and within the scope andrange of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 4 are diagrammatic, sectional views of a piston compressor infour successive piston positions during the starting operation of thepiston compressor according to the invention;

FIG. 5 is a graph of a characteristic profile of a load moment occurringduring the operation of the piston compressor according to FIG. 1, as afunction of the piston position; and

FIG. 6 is a graph according to FIG. 5 showing the profile of the loadmoment during the starting operation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In all the figures of the drawing, sub-features and integral parts thatcorrespond to one another bear the same reference symbol in each case.Referring now to the figures of the drawing in detail and first,particularly, to FIGS. 1-4 thereof, there is shown a piston compressor 1illustrated diagrammatically as a reciprocating-piston compressor. Thepiston compressor 1 contains a piston 2 which is displaceable linearly(in a vertical direction according to FIG. 1) in a pressure cylinder 3.For driving the piston 2, an electric motor 4 is provided, which, inparticular, is configured as a permanent-magnet synchronous motor and isactivated by an electronic control unit 5. The motor 4 acts via a driveshaft 6, merely indicated, on a drive eccentric 7 which, in turn, isconnected in an articulated manner to the piston 2 via a connecting rod8, so that the rotational movement of the drive eccentric 7 is convertedinto a linear oscillating movement of the piston 2 in a known way.

The pressure cylinder 3 and the piston 2 delimit the compression space9, into which an inlet 11 and an outlet 12 for a fluid F to becompressed issue on a cylinder bottom 10, facing away from the piston 2,of the pressure cylinder 3. The fluid F is preferably a refrigerant, forexample R600a.

At the respective issue of the inlet 11 and outlet 12 into thecompression space 9, an inlet valve 13 and outlet valve 14 are provided,which respectively shut off in a fluid-tight manner or release the inlet11 and the outlet 12, depending on the valve position. Each valve 13 or14 is configured as a flap which is prestressed into a closed positionof rest by an elastic force, in particular a spring, or due to theelastic configuration of the flap itself. Each valve 13 or 14 opens andcloses automatically, in the manner of a non-return valve, under theaction of a corresponding pressure gradient of the fluid F. The inletvalve 13 in this case opens when a sufficient under pressure prevails inthe compression space 9 with respect to the inlet 11. Similarly, theoutlet valve 14 opens when a sufficient over pressure prevails in thecompression space 9 with respect to the outlet 12.

The volume of the compression space 9 varies as a function of pistonposition S. In this case, the rotary or annular position of the driveshaft 6 and of the drive eccentric 7 connected fixedly in terms ofrotation to the latter is adopted as a measure of the piston position S.In a piston position S=0°, the piston 2 is moved into the pressurecylinder 3 to a maximum extent, and the volume of the pressure space 9is thus minimized. In a piston position S=180°, the piston 2 iscorrespondingly drawn out of the pressure cylinder 3 to a maximumextent, and the volume of the compression space 9 is thus maximized.Given rotation of the drive eccentric 7 in an operational direction ofrotation B indicated in FIG. 1, a change in the piston position S from0° to 180° corresponds to an expansion phase E (FIG. 5), in the courseof which the compression space 9 expands, and therefore fluid is suckedin from the inlet 11. A change in the piston position S from 180° to360°=0° corresponds to a compression phase K (FIG. 5), during which thefluid F contained in the compression space 9 is compressed and isexpelled through the outlet 12. The commencement of the expansion phaseE corresponding to a piston position S=0° is designated as thecompression point S_(k), and the commencement of the compression phase Kcorresponding to a piston position S=180° is designated as the expansionpoint S_(e). A full rotation of the drive eccentric 7, that is to say achange in the piston position S from 0° to 360°, is designated as thework cycle of the piston compressor 1. The work cycle thus contains theexpansion phase E and the subsequent compression phase K.

During the operation of the piston compressor 1, therearises—essentially due to the variable fluid pressure in the compressionspace 9—a load torque (designated below in brief as the load moment L)which is exerted on the drive eccentric 7 by the piston 2 and whichcounteracts a drive torque (designated below in brief as the drivemoment A) which is exerted on the drive eccentric 7 by the motor 4.

As illustrated by way of example in FIG. 5, the load moment L variesperiodically as a function of the piston position S. Under usualoperating conditions, the amount of the load moment L occurring duringthe expansion phase E is small, whereas, in the course of thecompression phase K, the load moment L passes through a pronouncedmaximum which is designated below as the load peak P. A maximum loadmoment L_(max) is reached in the region of the load peak P, for example,in a piston position S≈270°.

The motor 4 is configured in such a way that a maximum drive momentA_(max) capable of being exerted on the drive eccentric 7 is lower thanthe maximum load moment L_(max). Consequently, there is a pistonposition S, in which the load moment L just overshoots the maximum drivemoment A_(max), so that the motor power is no longer sufficient forfurther compression. The steady-state point S_(b), as it may be referredto, occurs in the vicinity of the load peak P, that is to say, when thepiston 2 is driven in the operational direction B, in a piston positionS which slightly undershoots 270°.

When the piston 2 is driven opposite to the operational direction ofrotation B, a load moment L′ (indicated in FIG. 3 as a dotted line)substantially mirror-symmetric with respect to the compression pointS_(k) and having an associated load peak P′ occurs in the pistoncompressor. During backward rotation, a corresponding steady-state pointS′_(b) is reached in a piston position S slightly overshooting 90°.

When the piston compressor 1 is in operation, the steady-state pointS_(b) and consequently the load peak P are overcome by the kineticenergy of the piston compressor 1 being utilized. However, the kineticenergy is not yet available during the starting of the piston compressor1, so that the piston compressor 1 cannot be started directly by itsinherent force from an initial piston position S_(I) illustrated by wayof example in FIG. 1. The method described in more detail below withreference to FIGS. 2 to 4 and 6 serves for improving the startingbehavior of the piston compressor 1.

The starting operation of the piston compressor 1 is accordingly dividedinto a resetting phase 15 (FIG. 6) and a subsequent acceleration phase16 (FIG. 6). During the resetting phase 15, the piston 2 is rotatedbackwards out of the initial piston position S_(I), opposite to theoperational direction of rotation B, with a low drive moment, until,with regard to the piston position S, a starting position S_(IV) (FIG.4) is reached. The piston compressor 1 in this case passes through thepiston positions S_(II) and S_(III) illustrated in FIGS. 2 and 3.

What is characteristic of the method according to the invention is thatthe starting position S_(IV) is selected in the vicinity of thecompression point S_(k) and in this case, in particular, between thesteady-state points S_(b) and S′_(b), so that, during the resettingphase 15, the steady-state point S′_(b) has to be overcome when thepiston 2 is rotated backwards. Likewise, if the piston 2 were to bemoved into the starting position S_(IV) by forward rotation, thesteady-state point S_(b) would have to be overcome.

The steady-state point S′_(b) is overcome in that, during the resettingphase 15, the motor 4 exerts a resetting drive moment A_(R) which isalso maintained precisely when the steady-state point S′_(b) is reachedor overshot. This is implemented in technical terms in that the statorfield of the motor 4 is rotated backwards at a low angular speed for apredetermined time span.

In this case, output points of the compression space 9, which, as aconsequence of construction, are formed, in particular, by thepiston/cylinder play always present to some extent, are utilized in acontrolled way. With the resetting drive moment A_(R) being maintained,fluid F (FIG. 3) escapes through the output points 17 to an appreciableextent, contrary to the usual operating behavior of the pistoncompressor 1, so that, under stationary load conditions, the volume ofthe compression space 9 is further reduced and the piston position Sthereby approaches the compression point S_(k) beyond the steady-statepoint S′_(b). The load peak P′ is virtually “tunneled through” in thisway.

Depending on the type of construction of the piston compressor 1, aleakage point which, if appropriate, connects the compression space 9 tothe inlet 11 may also be utilized additionally or alternatively to theleakage or output points 17 formed by the piston/cylinder play.Additionally or alternatively to leakage points 17 obtained as aconsequence of construction, an artificial leakage point, for example inthe form of a thin bypass line circumventing the inlet valve 13, mayalso be provided. This is expedient particularly when an escape of thefluid F into the environment is to be avoided.

The exact site of the starting position S_(IV) depends slightly on theinitial piston position S_(I). Motor activation during the resettingphase 15 is in this case configured in such a way that the startingposition S_(IV) always lies in an angular range of +/−30° about thecompression point S_(k) and, in an initial piston position S_(I) ofapproximately 180°, coincides substantially with the compression pointS_(k). It has proved expedient, for the suitable dimensioning of theresetting drive moment A_(R), to have a motor activation in which thestator field of the motor 4 is rotated backwards at an angular speed ofbetween 1 and 10 revolutions per minute for a predetermined time span ofbetween 3 and 30 seconds, in particular at approximately 4 revolutionsper minute for approximately 20 seconds.

After the starting position S_(IV) is reached, in the course of theacceleration phase 16 a starting drive moment A_(S) is exerted on thedrive eccentric 7, and therefore on the piston 2, the starting drivemoment acting in the operational direction of rotation B andcorresponding in terms of its amount to the maximum drive momentA_(max).

As can be seen particularly from FIG. 6, during the acceleration phase16 the piston position S passes through an acceleration range 18 of morethan 240°, in the course of which no increased load moment L occurs,with the result that the piston compressor 1 can absorb sufficientmomentum to overcome the load peak P. The long acceleration range 18thus makes it possible for the motor 4 to be configured with aparticularly low maximum drive moment A_(max), without the startingbehavior of the piston compressor 1 being impaired.

The resetting phase 15 and the acceleration phase 16 may be executeddirectly one after the other in time. However, the resetting phase 15and the acceleration phase 16 may also be executed separately from oneanother in time. In particular, there is optionally provision forcarrying out the resetting phase 15 at the end of an operating phase ofthe piston compressor 1, so that, during a subsequent operating phase,the piston compressor 1 is already in the starting position S_(IV) fromthe outset and can be started immediately by the execution of theacceleration phase 16. Alternatively to this, there is optionallyprovision for the method described above to be carried out only when apreviously conducted attempt at immediate starting has failed because ofan unfavorable initial piston position S_(I).

Where the piston compressor illustration in FIGS. 1 to 4 is concerned,furthermore, in a variant of the starting method described, a resettingdrive moment A_(R) acting in the operational direction of rotation B mayalso be exerted. Furthermore, the starting method described can also beapplied equivalently to a rotary-piston compressor.

The method described can also be used equivalently in a pistoncompressor with a different type of electronically regulated motor, inparticular a brushless direct-current motor (BLDC), an asynchronousmotor, a reluctance motor, etc.

This application also claims the priority, under 35 U.S.C. §119, ofGerman patent application No. 10 2004 057 467.7, filed Nov. 29, 2004;the entire disclosure of the prior application is herewith incorporatedby reference.

1. A method for starting a piston compressor, which comprises the stepsof: during a resetting phase, driving a piston of the piston compressorin a direction of a compression point of a piston position by applying aresetting drive moment; maintaining the resetting drive moment until thepiston reaches a starting position by overstepping a steady-state pointas a result of displacement of fluid out of a compression space, formedbetween the piston and a corresponding pressure cylinder, through atleast one leakage point; and during an acceleration phase, acceleratingthe piston out of the starting position into an operational direction ofrotation with a starting drive moment.
 2. The method according to claim1, which further comprises directing the resetting drive moment oppositeto the operational direction of rotation.
 3. The method according toclaim 1, which further comprises selecting the starting position in anangular range of the piston position of +/−30° with respect to thecompression point.
 4. The method according to claim 1, which furthercomprises exciting a stator field of a motor driving the pistoncompressor at a predetermined angular speed of between 1 and 10revolutions per minute for a predetermined time span of between 3 and 30seconds for generating the resetting drive moment.
 5. The methodaccording to claim 1, which further comprises setting said startingdrive moment to correspond to a predetermined maximum drive moment.
 6. Apiston compressor, comprising: a pressure cylinder; a drive shaft; amotor driving said drive shaft; a piston being moved in said pressurecylinder by said drive shaft being driven by said motor; and a controlunit for activating said motor, said control unit being configured to:in a resetting phase, predetermine a resetting drive moment, by whichsaid piston being driven in a direction of a compression point of apiston position; maintain the resetting drive moment until said pistonreaches a starting position by overstepping a steady-state point as aresult of displacement of fluid out of a compression space, formedbetween said piston and said pressure cylinder, through a leakage point;and in an acceleration phase, predetermine a starting drive moment, bywhich said piston is accelerated out of the starting position into anoperational direction of rotation.